Typical directional drilling procedures occasionally require that the drill string be turned at the surface in order to generate torque at the bottom thereof which will orient the bent housing in a manner so that the bit is steered azimuthally. The transmission of such torque can be done when a conventional drill pipe string is used, since it is quite rigid. An attractive alternative to drill pipe is coiled tubing which has been used in the past primarily in connection with well workover and repair operations, as well as stimulation. Coiled tubing has a relatively small size in the range of 3/4-27/8 inch, and a thin wall section of about 5/32 inch, which makes it flexible to the extent that many thousands of feet can be wound on a reel having a relatively small diameter in the order of 9-10 feet. Coiled tubing has the advantage over conventional drill pipe in that it can be run into and out of a well very quickly since there are no threaded joint connections to make up or break out, and the absence of threaded connections enables coiled tubing to be run while under pressure and while fluids are being pumped through it. However, coiled tubing has not heretofore been widely used to run a directional drilling tool string for the principle reason that is not possible to rotate coiled tubing at the surface to accomplish steering, on account of its storage on the reel. Thus it was thought that there was no effective way to steer the bit if coiled tubing is used as the running string.
It has been recognized that when a downhole motor is rotating the bit on bottom while weight (WOB) is being applied thereto, a reactive torque in the counterclockwise direction is applied to the housing of the motor, which includes the bent housing. The level of such counter-torque is directly proportional to the weight-on-bit, and has its maximum level at motor stall. Such reactive torque, and the presence of a bend point in the bent housing, causes lateral forces to be applied to the bit which tend to change the direction of the borehole. However, to control the direction, there must be a way to orient the bend point about the axis of the borehole. As noted above, this is accomplished when using a conventional drill pipe string by simply turning it at the surface. However, coiled tubing cannot be manipulated in this manner. The present invention provides a means and method of orienting the bent housing and its bend point downhole, which enables a directional drilling tool string to be run on coiled tubing.
In accordance with this invention, the drilling tool string includes a downhole adjustable orienting sub by which the relative angular orientation of the bend point established by the bent housing can be changed, as needed, to cause the bit to drill at a certain heading. Variations in the weight of the coiled tubing that is applied to the bit can be used to vary the level of the reactive torque and the resulting torsional wind-up angle of the bottom end of the coiled tubing, and also the lateral force on the bit, so that it will drill a borehole along a planned course. This drilling procedure also employs a measuring-while-drilling (MWD) tool that makes directional measurements and transmits signals representative thereof to the surface. Measurements are made of three orthogonal components of the earth's gravity field, from which the inclination of the borehole can be determined, and three orthogonal components of the earth's magnetic field from which azimuth of the borehole can be determined. These measurements, together with tool geometry, also permits "toolface" angle to be determined and displayed at the surface, along with the inclination and azimuth values while drilling is in progress. As used herein, the term "toolface" means the orientation angle of the bent housing or sub in the borehole with respect to a reference such as high side of the borehole which indicates the direction in which the borehole will be curving.
The general object of the present invention is to provide a directional drilling tool string of the type described which is run on coiled tubing and which includes an orientation sub that can be adjusted downhole to fix the angular orientation of the bend point in the bent housing or sub with respect to the axis of the borehole so that the direction of the borehole can be controlled.
Another object of the present invention is to provide a directional drilling tool string of the type described where the bent housing can be oriented downhole to various angular positions, and where the amount of weight-on-bit can be varied to change the reactive torque and wind-up angle in a manner such that a directional hole having a desired trajectory will be drilled.